In the field of semiconductor manufacturing, it has been recognized since the beginning of the industry that removing particles from semiconductor wafers during the manufacturing process is a critical requirement to producing quality profitable wafers. While many different systems and methods have been developed over the years to remove particles from semiconductor wafers, many of these systems and methods are undesirable because they damage the wafers. Thus, the removal of particles from wafers, which is often measured in terms of the particle removal efficiency (“PRE”), must be balanced against the amount of damage caused to the wafers by the cleaning method and/or system. It is therefore desirable for a cleaning method or system to be able to break particles free from the delicate semiconductor wafer without resulting in damage to the devices on the wafer surface.
Existing techniques for freeing the particles from the surface of a semiconductor wafer utilize a combination of chemical and mechanical processes. One typical cleaning chemistry used in the art is standard clean 1 (“SC1”), which is a mixture of ammonium hydroxide, hydrogen peroxide, and water. SC1 oxidizes and etches the surface of the wafer. This etching process, known as undercutting, reduces the physical contact area of the wafer surface to which the particle is bound, thus facilitating ease of removal. However, a mechanical process is still required to actually remove the particle from the wafer surface.
For larger particles and for larger devices, scrubbers have historically been used to physically brush the particle off the surface of the wafer. However, as devices shrank in size, scrubbers and other forms of physical cleaning became inadequate because their physical contact with the wafers began to cause catastrophic damage to the smaller/miniaturized devices.
Recently, the application of sonic/acoustical energy to the wafers during chemical processing has replaced physical scrubbing to effectuate particle removal. The terms “acoustical” and “sonic” are used interchangeably throughout this application. The acoustical energy used in substrate processing is generated via a source of acoustical energy, which typically comprises a transducer which is made of piezoelectric crystal. In operation, the transducer is coupled to a power source (i.e. a source of electrical energy). An electrical energy signal (i.e. electricity) is supplied to the transducer. The transducer converts this electrical energy signal into vibrational mechanical energy (i.e. sonic/acoustical energy) which is then transmitted to the substrate(s) being processed. Characteristics of the electrical energy signal, which is typically in a sinusoidal waveform, supplied to the transducer from the power source dictate the characteristics of the acoustical energy generated by the transducer. For example, increasing the frequency and/or power of the electrical energy signal will increase the frequency and/or power of the acoustical energy being generated by the transducer.
Over time, wafer cleaning utilizing acoustical energy became the most effective method of particle removal in semiconductor wet process applications. Acoustical energy has proven to be an effective way to remove particles, but as with any mechanical process, damage is possible and acoustical cleaning is faced with the same damage issues as traditional physical cleaning methods and apparatus. In the past, cleaning systems utilizing acoustical energy were designed to process semiconductor wafers in batches, typically cleaning twenty-five substrates at once. The benefits of batch cleaning became less important as the size of substrates and the effectiveness of single-wafer cleaning systems increased. The greater value per semiconductor wafer and the more delicate nature of the devices resulted in a transition in the industry toward single-wafer processing equipment.
An example of a single-wafer cleaning system that utilizes megasonic energy is disclosed in U.S. Pat. No. 6,039,059 (“Bran”), issued Mar. 21, 2000, and U.S. Pat. No. 7,100,304 (“Lauerhaas et al.”), issued Sep. 5, 2006, the entireties of which are hereby incorporated by reference herein. The single-wafer cleaning system that is the subject of U.S. Pat. No. 6,039,059 and U.S. Pat. No. 7,100,304 is commercialized by Akrion, Inc. of Allentown, Pa. under the name GOLDFINGER®. Other examples of single-wafer cleaners that utilize acoustic energy are disclosed in U.S. Pat. No. 7,145,286 (“Beck et al.”), issued Dec. 5, 2006, U.S. Pat. No. 6,539,952 (“Itzkowitz”), issued Apr. 1, 2003, and United States Patent Application Publication 2006/0278253 (“Verhaverbeke et al.”), published Dec. 14, 2006. In single-wafer acoustic cleaning systems, such as the ones mentioned above, a semiconductor wafer is supported and rotated in horizontal orientation while a film of liquid is applied to one or both sides/surfaces of the wafer. A transducer assembly is positioned adjacent to one or the surfaces of the wafer so that a transmitter portion of the transducer assembly is in contact with the film of liquid by a meniscus of the liquid. The transducer assembly is activated during the rotation of the wafer, thereby subjecting the wafer to the acoustic energy generated by the transducer assembly.
Nonetheless, the industry's transition to the below 100 nm devices has resulted in additional challenges for manufacturers of semiconductor processing equipment. The cleaning process is no different. As a result of the devices becoming more and more miniaturized, cleanliness requirements have also become increasingly important and stringent. When dealing with reduced size devices, the ratio of the size of a contaminant compared to the size of a device is greater, resulting in an increased likelihood that a contaminated device will not function properly. Thus, increasingly stringent cleanliness and PRE requirements are needed. As a result, improved semiconductor wafer processing techniques that reduce the amount and size of the contaminants present during wafer production are highly desired.
As a result of these increasingly stringent cleanliness and PRE requirements, the removal of particles from both sides/surfaces of the wafer have been discovered by the present inventors to be playing an increasingly important role in achieving high yields. In existing single-wafer systems, removal of particles from both surfaces of the semiconductor wafer during a cleaning cycle are achieved by providing a single transducer assembly adjacent to one of the surfaces of the wafer. This transducer assembly is operated at a sufficient power level so that the generated acoustic energy passes through the wafer itself to loosen particles on the opposite surface of the wafer. This basic concept is one of the subject inventions of U.S. Pat. No. 6,039,059. This dual-sided cleaning concept is also shown as being utilized and copied in the system disclosed in United States Patent Application Publication 2006/0278253 (“Verhaverbeke et al.”) with the transducer assembly located adjacent the backside of the wafer.
Despite these advancements in single-wafer systems and methods for cleaning both sides of the wafer, there still remains a need for single-wafer systems that can achieve improved PRE with minimized device damage. Furthermore, the continued miniaturization of devices continues to render existing cleaning systems incapable of achieving an acceptable balance between high PRE and minimized device damage.